Natural gas and fuel oil serve as the energy source for much of the currently generated electricity. To this end, the gas or fuel oil undergoes combustion in a turbine which powers an electrical generator. The products of combustion leave the turbine as an exhaust gas quite high in temperature so that the exhaust gas represents an energy source in itself. This energy is captured in a heat recovery steam generator (“HRSG”) that produces superheated steam that powers another electrical generator.
Generally, an HRSG comprises a casing having an inlet and an outlet and a succession of heat exchangers—that can include a superheater, an evaporator, and an economizer arranged in that order within the casing between the inlet and outlet.
Such heat exchangers for an HRSG can have multiple banks of coils, the last of which in the direction of the gas flow can be a feedwater heater. The feedwater heater receives condensate that is derived from low-pressure steam discharged by the steam turbine, and elevates the temperature of the water. Then the warmer water from the feedwater heater flows for example into one or more economizers, boiler feed pumps or evaporators, which convert it into saturated steam. That saturated steam flows on to a superheater which converts it into superheated steam. From such a superheater, the superheated steam can flow to the steam turbine.
Generally, in the above-discussed process, most HRSGs produce superheated steam at three pressure levels—low pressure (LP), intermediate pressure (IP) and high pressure (HP). An HRSG can thus have one or more superheaters and also can have what are termed an LP Evaporator, an HP Economizer, and an IP Economizer.
An overall illustration of a system which features an HRSG using a natural circulation system appears in U.S. Pat. No. 6,508,206 B1 (hereafter “'206 Patent”), which '206 Patent is incorporated by reference. FIG. 4 of the '206 Patent illustrates an arrangement with a superheater 18 located at the farthest position upstream. Downstream from the superheater 16 in the internal HRSG flow path is at least one evaporator 18 which has in fluid flow connection therewith a steam drum shown located atop of the evaporator. That steam drum is located outside of the HRSG internal exhaust gas flow path. The HRSG in the '206 Patent also has a feedwater heater 20.
The superheated steam produced by an HRSG has typically been below the critical point pressure of steam. Industry trends to build power plants of larger scale and of greater efficiency have evolved into a need for such plants to operate above, or just below, the critical pressure of water.
In a natural circulation HRSG, water is first evaporated into saturated steam. This takes place in the high pressure (HP) evaporator coil and drum combination, which is simply referred to herein as “HP evaporator section” (HPEVAP). In such an HP evaporator coil and drum combination, the evaporator coil is located within the internal exhaust flow path of the HRSG, while the drum is located exterior to the internal exhaust flow path of the HRSG, with the HP evaporator coil and drum being in fluid flow connection with one another. In the HPEVAP, the density difference of steam and water at saturation conditions is the driving force to cause water and/or steam to circulate from a steam drum through downcomer pipes to the HPEVAP coil tubes, and through risers back to the steam drum. This circulation of saturated water in the HPEVAP is what distinguishes a natural circulation HRSG from other types of HRSGs.
Another type of HRSG is a system that uses a once-through steam generator, commonly referred to in the art as an “OTSG”. In an OTSG, the working fluid does not recirculate through the heating surface as with a natural circulation HRSG system. Rather, with an OTSG the working fluid makes one pass through each individual parallel HPEVAP conduit and then exits the OTSG. U.S. Pat. No. 6,019,070 to Duffy (“Duffy ‘070’ Patent”) discloses an HRSG having an OTSG with what are designated therein as circuit assemblies. Those circuit assemblies in the Duffy '070 Patent each comprises a serpentine shaped heat exchange tube with U-bend shaped portions and vertically oriented linear portions, positioned within the HRSG internal gas flow path.
U.S. Pat. No. 6,189,491 to Wittchow, et al. (“Wittchow '491 Patent”) also discloses an HRSG having an OTSG with vertically disposed steam-generator tubes within the HRSG gas flow path. U.S. Pat. No. 8,959,917 to Berndt, et al. (“Berndt '917 Patent”) discloses an HRSG that uses an OTSG, while U.S. Patent Application Zhang having Pub. No. US 2013/0180228 A1, discloses an HRSG with a supercritical evaporator arrangement (“Zhang '228 Applic.”) The said Duffy '070 Patent, Wittchow '491 Patent, Berndt '917 Patent and Zhang '228 Applic. are incorporated herein as if fully set forth herein.
FIG. 1 of the present application shows an overall layout of a system that illustrates use of an HRSG similar to that shown in FIG. 3 of the '206 Patent. FIG. 1 of the present application discloses a gas turbine G that discharges hot exhaust gases into an “HRSG”, which extracts heat from the gases to produce steam to power a steam turbine S. The gas turbine G and steam turbine S power the generators E that are capable of producing electrical energy. The steam turbine S discharges steam at a low temperature and pressure into a condenser CN, where it is condensed into liquid water. The condenser CN is in flow connection with a condensate pump CP that directs the water back to the HRSG as feedwater.
Generally, the heat exchangers comprise coils that have a multitude of tubes that usually are oriented vertically and arranged one after the other transversely across the interior of the casing. The coils are also arranged in rows located one after the other in the direction of the hot gas flow depicted by the arrows in FIGS. 2-7 of the present application. The tubes contain water in whatever phase its coils are designed to accommodate. The length of the tubes can be as great as about 90′ tall.